With the accelerating energy consumption of conventional fossil fuels such as coal, oil, natural gas, etc., the eco-environment is continuously deteriorating. Especially, due to the greenhouse gas emissions, the global climate change is getting worse and worse. Therefore, the sustainable development of human society has been seriously threatened. Countries all around the world have formulated their own energy development strategies to cope with the limitations of conventional fossil fuels resources and the environmental problems caused by the development and utilization of the conventional fossil fuels. Solar energy has become one of the most important renewable energy sources due to its reliability, safety, universality, long service life, environmental friendliness and resource sufficiency; and is expected to become the dominant energy of the future global electricity supply.
Considering the fact that green solar energy has been promoted and used vigorously, the components of the photovoltaic modules are encapsulated based on the electrical principle of low current and low loss (i.e., the power loss of photovoltaic modules is directly proportional to the square of the working current) through the slicing process (i.e., performing one-half laser cutting on conventional 6-inch solar cells) to obtain two separate complete power generating units, so as to increase the output power and electricity generating efficiency. Currently, the first-class photovoltaic module manufacturers in China such as Trina Solar, Canadian Solar, Jinko Solar, and JA Solar have all launched half-cell photovoltaic modules. Among them, manufacturers like Canadian Solar etc. have already sold the half-cell components in large numbers in the market and have earned an objective sales profit. Based on the consideration of the characteristics of the assembly line of existing manufacturing process, most of the internal protection circuits of the conventional half-cell photovoltaic modules available in the market are designed such that two strings are connected in parallel, and then three strings are connected in series. Otherwise, the two strings are connected in parallel, and then five strings are connected in series, wherein a plurality of diodes are connected in bypass to form a bus circuit (taking the 60-cell mode for an example). The final product, i.e. the half-cell photovoltaic modules, can achieve almost the same output voltage and current as those of the conventional photovoltaic modules, without significant impact or extra cost on the system-side application. Moreover, compared to the conventional product, the output power gain of the final product increases by about 5 watts, and the efficiency thereof increases by about 0.3%.
Generally, the 60-cell mode is taken as an example for the design of internal protection circuit of the existing half-cell photovoltaic modules. If the vertical outgoing is taken for an example, two strings are connected in parallel, and then three strings are connected in series, wherein two or four diodes are connected in bypass. If the lateral outgoing is taken for an example, two strings are connected in parallel, and then five strings are connected in series, wherein four diodes are connected in bypass. Considering the factors such as the compatibility of the assembly line of the manufacturing process of the conventional photovoltaic modules, the performance and reliability of the final product, etc., the above two kinds of internal protection circuit designs have some defects and drawbacks.
Taking the vertical outgoing as an example (i.e. 20 half-cells are connected to each other to form a substring power generating unit), two bypass diodes are used to protect the solar cells. If one substring in three substrings is shadowed to be put in the reverse bias state, the diodes connected in parallel to the shadowed substring are turned on and start to work. However, in reality, only one substring (40 half-cells in total) works effectively to generate electricity. Two-third of the substrings of the solar cell (80 half-cells in total) are bypassed such that the power generation capacity thereof is lost. In this case, the actual utilization rate of electricity generation of the half-cells is about 33% in total. The obvious disadvantages are as follows. In view of the electrical power output of the product, the levelized cost of electricity (LOCE) of the system side application of photovoltaic modules will be seriously affected. Moreover, the risk of failure in the reliability test of the products will be greatly increased. Therefore, the performance of the products is unstable, and the products are likely to fail in the environment test such as the typical hot spot test.
Taking the lateral outgoing as an example (i.e. 12 half-cells are connected to each other to form a substring power generating unit), four bypass diodes are used to protect the solar cells. If one substring in five substrings is shadowed to be put in the reverse bias state, the diodes connected in parallel with the shadowed substring are turned on and start to work. At this time, actually, only three-fifth substrings (72 half-cells in total) work effectively to generate electricity. Two-fifth of the substrings of the solar cell (48 half-cells in total) are bypassed such that the power generation capacity thereof is lost, and the actual utilization rate of electricity generation of the half-cells is about 60%. The obvious disadvantages are as follows. In view of the electrical power output of the product, compared to the above vertical outgoing method, the power generation capacity of the lateral outgoing method increases by about 27%. However, the actual power generation capacity of the system side of the modules is greatly reduced. Meanwhile, the failure rate of the environment test is also increased.